Great thread, it's actually a good insight to see what are other people's go-to instrument and effects.
Instruments:
Blamsoft's Distributor: Okay, this instrument might not make any sound, but why is it at the top of my list? It's the most used because I hate standard tuning and not having deep control over each voice. I'm always building microtonal paraphonic synths and if you don't as well then I'll likely never listen to your music.
RS's Europa: I actually feel bad using this synth all the time because how dirty RS's stabbed the back of existing developers when making this. I'd love to use
Expanse instead but it's never on sale!
RS's Grain: I really should be using the other granular synthesizer RE's I own but for some reason I keep going back to this, might be because it has a decent UI. My only gripe with the Reason RE devices is that their internal LFO only goes down to 0.050 Hz, so disappointing.
RS's Complex-1: This is my go-to for Sub-basslines and noise generating/shaping. It actually sounds pretty decent with it's internal oversampling.
I don't really have a 5th because I switch a lot between Mimic, Subtractor, Antidote, Subtractor, etc.
Notable mentions RX1200 for it's crunchy 12 bit modeling but I find myself using it less and less because there is no attack control. Not complaining because the original didn't have it either, maybe I should adjust the attack on another unit and resample into this one...
Effects:
McDSP's Moo Tube Compressor: I have no idea why but I find myself using this compressor on almost everything. I know it's a tube compressor but it sounds really transparent and subtle, yet straight-forward and powerful. I use it surgically to fix technical problems and even creatively in parallel (it's fast/zero latency!)
SKP Sound Design's Sugar-Q Sweetening Equalizer: What can I say, there is really something special about this RE. The mojo sounds like nothing else, maybe one of my favorite subtle-yet-powerful saturating algo's in the RE world (aside from Softube's Saturation device). The EQ sections is really handy for curtailing your sound and I really like the 28kHz low pass when trying to "box-in" unruly synthesizers. It's shelf parameters are also really handy and good-sounding for basic corrections or enhancing tonal qualities.
Sonic Charge's Echobode: Yeah... yeah... there are a million delay devices in the shop, but for some reason I find myself always gravitating towards this. I love the subtle stereo effect you can get by modulating the frequency domain. It also sounds great when modulated with CV. It can do everything from subtle to otherworldly.
Synapse's AF-4 Analog Filter We've needed something like this in Reason for a long... long time. It's my go-to analog filter and the added audio inputs to modulate the FM really make this like no other. I wish Synapse would release another analog filter modeled after other units. It doesn't work for everything but if you set the right limitation and tune it properly, it can be an amazing effect and even sound-shaping tool. I sometimes use it like an EQ (steep-shelf filter with resonance boost).
As with instruments, there are a lot of units that I can't really pick a 5th. Honorable mentions would be Blamsoft's Filter (not F-16, the pultec series one), Neutron
Particle Emitter by LoveOne Consulting AB, and I'm really proud/like what Dusty Devices are doing with their Ensoniq phaser emulation, the incredible vinyl emulation and that sweet sounding tilt filter.
I really like that developers like Turn2On and Ekssperimental Sounds are pumping out a ton of RE's but I think their products lack some magic or are too gimmicky.